How do I change the mode of perspective?
If this is what you're asking, you can switch from User Perspective to Orthographic by using the Numpad 5 key.  The perspective type is given in the upper left hand corner of the 3D view.

The differences can be found here: What are the differences between Orthographic and Perspective views?

Essentially, Orthographic is a "non-real-world" view in which all dimensions have the same scale... i.e. objects that are farther away appear to be the same size as objects that are near-field.